The invention is directed to a single mode optical waveguide fiber wherein a refractive index profile design is optimized for high data rate links, or systems using optical amplifiers, or wavelength division multiplexed systems.
The full capability of optical waveguide fiber is being exploited by high data rate systems having a long distance between repeaters. The operating window in a range including 1550 nm is attractive for these systems because of the lower attenuation possible and the absence of absorption peaks. Data rates typical of such systems are greater than 1 gigabit/sec and repeater spacing exceeds 50 km.
The high data rates require that the birefringence of the waveguide fiber be low. That is, the dispersion of the polarizations of the single propagated mode must be controlled to limit bit errors. The high data rates also require that the zero dispersion wavelength be near 1550 nm to limit material dispersion. Furthermore, the introduction of high powered lasers has produced non-linear effects which can limit data rate or repeater spacing. In systems which utilize wavelength division multiplexing over a relatively small wavelength range, the non-linear interference effect called four wave mixing (FWM) is especially detrimental.
One approach to limiting polarization mode dispersion (PMD) is to provide a waveguide fiber which is relatively free of birefringence. This may be accomplished by maintaining circularly symmetric geometry and by limiting residual stress in the fiber. In addition, a waveguide having a relatively lower dopant level in the signal carrying portion of the waveguide will have reduced Rayleigh scattering and will reduce bit errors due to non-linear effects.
The impact of non-linear effects can also be lessened by providing a larger mode field diameter to reduce power density in the waveguide fiber. Four wave mixing can essentially be eliminated by moving the zero dispersion wavelength out of the operating window. A non-zero dispersion over the operating window serves to prevent the phase matching of multiplexed signals thereby eliminating the four wave mixing signal interference.
The objectives, therefore, in manufacturing a waveguide fiber for high data rate, long repeater spacing and multichannel operation are to provide:
low residual stress;
reduced overlap of signal with higher dopant waveguide regions;
higher modefield; and,
dispersion zero away from the operating window.
Further, these properties must be achieved while maintaining low attenuation, acceptable bend performance and appropriate cut off wavelength. An added benefit can be realized if the performance goals can be met without increasing manufacturing difficulty or cost.
The present invention fulfills the requirements for a high performance waveguide fiber. Further, a waveguide of the inventive design is relatively easier to manufacture and thus is lower in manufacturing cost.
A major feature of this invention, which distinguishes it from other compound core profile designs, is that a central core region is maintained relatively low in dopant content. This central region is adjacent to a region relatively higher in dopant content. The advantageous result is a profile design flexible enough to satisfy an exacting specification but simple enough to allow ease of manufacture using standard equipment. The inventive profile effectively controls index on centerline and moves the index peak to an off centerline position.
A first aspect of the invention is a single mode optical waveguide fiber having a compound core. A central region of the core has a minimum refractive index n0 and a radius a0. The central core region is surrounded by at least one annular core region where the innermost of the annular regions has a minimum refractive index ni and a radius ai and where ni greater than n0 and ai greater than a0. The core is surrounded by a cladding layer having refractive index nc where ni greater than nc. The highest index point of the central core region may occur at or near the waveguide axial centerline.
In general, the refractive index of the central region and the refractive indices of the at least one surrounding annular region may vary with radius. A preferred embodiment of the inventive refractive index profile is one in which the refractive index in each core region is essentially cylindrically symmetrical.
In another preferred embodiment, the waveguide profile is essentially cylindrically symmetric and the core comprises one annular region surrounding the central core region.
A most preferred embodiment has a cylindrically symmetric waveguide refractive index and a core refractive index profile including a substantially constant index over a single annular region surrounding the central core region. The central core region index may also be substantially constant in this embodiment. Further, the central core region index may be substantially equal to the refractive index of the cladding, i.e., the central core % delta is inside the range +/xe2x88x920.1%.
Also contemplated are designs which reduce the refractive index, relative to the refractive index of silica, of all or part of any of the core regions or all or part of the clad layer.
Another aspect of the invention is a waveguide fiber having a central core surrounded by two annular regions having respective minimum refractive indices n1 and n2. The first annular region is adjacent the central core region and the second annular region surrounds and is adjacent to the first annular region. The relationship of the refractive indices of the respective regions is n1 greater than n0 and n2 greater than n0, where n0 is the central core region minimum refractive index.
A further aspect of the invention is a single mode optical waveguide fiber including a central core region having a substantially constant refractive index n0. The central core region is surrounded by at least one annular region. The annular region adjacent the core has minimum refractive index ni, where ni greater than n0. The waveguide has a clad layer having refractive index nc surrounding the core region.
In a preferred embodiment, the substantially constant refractive index of the central core region is substantially equal to the refractive index of the clad layer. In this embodiment the total dispersion slope can be less than about 0.05 ps/nm2/km. The maximum dispersion slope of this embodiment is typically no greater than 0.075 ps/nm2/km. The embodiment is relatively free of draw induced residual stress and stress due to thermal expansion mismatch. In addition, the zero dispersion wavelength is relatively insensitive to changes in cut off wavelength or core diameter. A change of about 5% in either cut off wavelength or core diameter produces substantially no change in zero dispersion wavelength. Furthermore, in this embodiment the zero dispersion wavelength can be moved away from the operating wavelength range to a wavelength less than about 1530 nm or greater than about 1565 nm.
Yet another aspect of the invention is a single mode optical waveguide fiber including a core having an axially symmetric central region of minimum refractive index n0 surrounded by an axially symmetric annular region of minimum refractive index n1, an inner radius ai and an outer radius ao, where n1 greater than n0 and the ratio ai/ao is in the range of about 0.35 to 0.80. The core is surrounded by a clad layer of refractive index nc, where n1 greater than nc.
In preferred embodiments of this aspect, n0 is substantially constant, or n0 is substantially equal to nc, or n1 is substantially constant. A preferred value of n1 is in the range of about 1.4700 to 1.4800.
Other features and advantages of the inventive refractive index profile will be described in the detailed description in conjunction with the following drawings.